plugins: imap-filter-sieve: SIEVE SCRIPT - Cleanup istream return value checking

The old code seems to have worked correctly already, but now it's a bit
clearer how it was intended to work.

src/plugins/imap-filter-sieve/cmd-filter-sieve.c

@@ -205,7 +205,12 @@ cmd_filter_sieve_script_read_stream(struct imap_filter_context *ctx)
 
 	while ((ret = i_stream_read_more(input, &data, &size)) > 0)
 		i_stream_skip(input, size);
-	if (input->v_offset == ctx->script_len) {
+	if (ret < 0) {
+		if (input->v_offset != ctx->script_len) {
+			/* client disconnected */
+			i_assert(input->eof);
+			return -1;
+		}
 		/* finished reading the value */
 		i_stream_seek(input, 0);
 
@@ -218,10 +223,6 @@ cmd_filter_sieve_script_read_stream(struct imap_filter_context *ctx)
 		i_stream_unref(&ctx->script_input);
 		return 1;
 	}
-	if (input->eof) {
-		/* client disconnected */
-		return -1;
-	}
 	return 0;
 }